Dimming LEDs

[BobM]

I know LEDs say they are not dimmable, but is that due to shortening their life expectancy, or does it physically damage them or is it dangerous?

Might seem a silly question, but most of our fittings are on dimmers, and saving teh planet seems like a good idea so LEDs seem the way to go.

 

[bernardgreen]

I know LEDs say they are not dimmable,

LEDs as in the actual Light Emitting Diode are dimmable and dimming them extends their all ready long life time.

What are not always "dimmable" are the power supplies refered to as "electronic transformers" that provide the LEDs with their required voltage supply.

 

[aptsys]

Most of the low power LED replacements for mains halogen lamps usually just use a capacitor to limit the current (along with an inrush current limiting resistor). Altering the voltage using a variac will successfully dim these bulbs, but they don't respond well to domestic type dimmers and tend to cause lots of smoke.

Also, unless you have a massive array of these fittings, you won't achieve the minimum rating of the dimmers switch.

The best option is to use high power LEDs along with a constant current source with 0-10V input for brightness.

 

[tim west]

Not sure a variac approach is the right way,The most common form of dimming Leds and other discharge lights such as a neon is through Pulse Width Modulation or PWM. The duration of On as opposed to off determines how bright it is.

 

[aptsys]

tim, PWM or current controlled dimming are the best method of choice for dimming LEDs, but I assumed the OP was referring to dimming the GU10 replacements where this type of brightness control would be unsuitable because of the regulation circuitry incorporated into the 'bulb'.

 

[tim west]

Yes apologies
mind you read this:

the variac may still cause problems by affecting the circuit which uses capacitance to alter the phase angle to lead and the inductance of a variac may affect this.

 

[plugwash]

a variac should provide an output that is very much like the mains but at a lower voltage. Sure there might be a bit of series inductance in the output but I can't imagine that causing any problems.

sadly variacs are damn bulky so using them for domestic dimming isn't very practical.

 

[tim west]

a variac should provide an output that is very much like the mains but at a lower voltage. Sure there might be a bit of series inductance in the output but I can't imagine that causing any problems.

sadly variacs are damn bulky so using them for domestic dimming isn't very practical.

if it affects the angle in any way then the voltage could go up. or are you saying the decrease in voltage from the variac could negate any increase due to difference in angle?

 

[plugwash]

what matters is the voltage and waveform at the terminals of the LED drive circuit, angles only mean anything when comparing two synchronised waveforms not when looking at any one waveform (in this case the supply to the led drive circuit) in isolation.

now series inductance combined with a highly capacitive load can indeed boost the voltage a bit but i think it is highly unlikely that this affect would be significant. The output impedance of a varic is likely to be in the range of ohms while the imput impedance of a capacitive LED driver circuit is up in the kilohms. Affects of the order of 0.1% (hell even those on the order of 1%) can generally be safely ignored.

 

[breezer]

interesting reading, but there has to be more to it than that. (dimming them)
I base this on the fact that i have dmx operated led "flood light"
yes it produces umpteen million colours, and yes it does dim the leds to do this (rgb leds) but the dmx bit cant be that big, so why is the whole pcb so big?

no i cant get a good pic of the pcb becuse its still inside the light but i can move it around and just about see inside.

also i know the makes have to make money but why are they still so expensive (if its easy to do)

 

[plugwash]

LEDs used for lighting (as apposed to the more familiar type used for indicators) mean high currents and low voltages, to get a good illusion of dimming by varying the duty cycle means reasonablly fast switching. Fast switching of high currents can be a pain.

plus I suspect a good part of it is that dimmable LEDs are currently a niche market. niche markets mean high prices (I heared one person estimate that as a small buisness making electronics you shouldn't even consider introducing a product at less than three times it's construction cost!)

in many areas of development there is a big chicken and egg problem, prices won't come down until something is mass market but something won't become mass market until prices come down.

 

[tim west]

what matters is the voltage and waveform at the terminals of the LED drive circuit, angles only mean anything when comparing two synchronised waveforms not when looking at any one waveform (in this case the supply to the led drive circuit) in isolation.

I cant agree looking at the circuit again the Reactive capacitance is important, therefore putting a great big coil in circuit with it must affect the Xc

 

[plugwash]

Xl does indeed subtract from Xc when you put an inductor and capacitor in series but the series inductive reactance of the output of the variac is going to be far far lower than the capactive reactance of the dropping capacitor so the affect will be negligable.

remember a variac is a type of transformer (it is an autotransformer with a way to vary the position of the output tap), with a transformer most of the effective inductance ends up accross the supply not in series with the load.

 

[bernardgreen]

In front of me I have a low cost ""dimmer"" for LED lighting. The lights supplied with it lasted about 3 months before the first failed and within a year half the LEDs had failed.

The dimmer works in a way that will seriously shorten the life of the LEDs.

Most LEDs can be pulsed with a current higher than the continuous design current ( Icon) . Switch the LED on for a tenth of a second at 10 times Icon and then off for 9/10th and the average current is Icon but because the eye sees the 10 times brighter flash the LED looks much brighter then if it were run at Icon continuously. This method is used as power saving for battery operated LED warning lights thathave to be very bright.

The normal maximum is 10 times Icon but some LEDs are designed for up to 16 times Icon provided the average current is not greater than that which the LED is designed for.

This dimmer is producing at full brightness a pulse that is 20 times the current the supplied LEDs can safely take. The pulse is very short so the average current is very low but the high pulse current will damage the LED molecule by molecule.